The omega-3 (or n-3) polyunsaturated fatty acid known as docosahexaenoic acid (DHA) is found in abundance in the brain and retina. DHA has been found to be required at high levels in the brain and retina for optimal mental functioning (learning ability, etc.) and visual acuity (visual performance), respectively. Therefore, dietary DHA is regarded as an essential dietary nutrient for infants (both preterm and term) and children on mixed diets ((British Nutrition Foundation: Unsaturated fatty acids: Nutritional and Physiological Significance. Andover, England: Chapman and Hall, 1992, and British Nutrition Foundation; Recommendations for Intakes of Unsaturated Fatty Acids, 1992).
Studies have shown significant alleviation of various risk factors for cardiovascular disease (CVD) when fish and fish oils containing the omega-3 polyunsaturated fatty acids DHA and eicosapentaenoic acid (EPA) are consumed. Epidemiological studies have also shown an inverse relationship between DHA levels in the population (diet/blood) and the risk of CVD (Leng, G. C. et al., Arterioscler. Thromb. 1994; 14:471-478, Simon, J. A., et al. Am. J. Epidemiol. 1995; 142:469476). Therefore, part of the cardioprotective effect of fish/fish oils containing omega-3 polyunsaturated fatty acids is due to DHA, in addition to EPA. The cardioprotective effects of omega-3 fatty acids are considered to be mediated by a number of physiological/biochemical mechanisms. Studies have shown that the enrichment of heart tissue in DHA provides an antiarrhythmic effect (Pepe, S. and McLennan, P. L. J. Nutr. 1996; 126:34-42) which may account for the reductions in cardiac arrest and sudden cardiac death in those having a higher DHA status (in diet/body). In addition, dietary DHA intakes and increased status in the body have been implicated in favourable effects on attention-deficit disorders (Stevcns, L. J. et al Am. J. Clin. Nutr. 1995; 62:761-768), depression and anxiety disorders (Hibbeln, J. R. and Salem, N. Am. J. Clin. Nutr. 1995; 62:1-9.), as well as protection against breast cancer in postmenopausal women (Zhu, Z. R., et al., Nutr. Cancer. 1995; 24:151-160).
DHA is found In abundance in fish and fish oils (with very minor amounts in eggs and some meats), but it is absent from all plant-derived food products including vegetable oils. Certain plant oils (eg., soybean oil) contain moderate amounts of another omega-3 fatty acid, alpha-linolenic acid (LNA), which can, to an extremely limited extent (Emken, E. A., et al. BBA. 1994; 1213:277-288 and Salem, N., et al., Proc. Natl. Acad. Sci. 1996; 93;49-54.) be metabolically converted in the body to provide sub-optimal amounts of DHA.
Mothers' breast milk in North America and elsewhere typically contains approx. 0.14 to 0.2% by wt. of the fat/fatty acids as DHA (Chen, Z.-Y., et al., Lipids. 1995; 30:15-21 and Makrides, M., et al., Am. J. Clin. Nutr. 1995; 61:1231-1233). Infants on breast milk (a source of DHA) perform better in visual acuity testing than those on formulae typically used in North America lacking DHA (Nettleton, J. A. J. Am. Diet Assoc. 1993; 93:58-64 and Makrides, M., et al., Lancet. 1995; 345:1463-68). Intelligence scores were also found to be higher in children receiving breast milk containing DHA when young (Lucas, A., et al., Lancet. 1992; 339:261-264). DHA has been added to selected infant formula products in Japan and recently in Europe to provide a direct dietary source of DHA in infant formula thereby providing optimal DHA levels and its associated benefits to infants. (JP 043411; WO 9212711; EP 404058; U.S. Pat. No. 4,670,285). Unfortunately, cow's milk is devoid of DHA and has only very modest amounts of LNA. Consequently, the recommended dietary levels of DHA for children on mixed diets (see recommendations of the British Nutrition Foundation; Recommendations for Intakes of Unsaturated Fatty Acids, 1992) cannot be fulfilled with cow's milk and associated dairy products (cheeses, ice-creams, etc.). This is particularly important at a development stage where there is active learning, information processing, and intellectual development.
Efforts have been made to increase dietary intake of DHA by adding or incorporating DHA into various foods. Methods have been developed to increase the level of omega-3 fatty acids in the flesh of beef cattle (U.S. Pat. No. 5,290,573), sows (U.S. Pat. No. 5,106,639; DE 3808885; Taugbol, O. et al., Zentralbl. Veterinarmed. A., 40(6): 437-443, 1993), poultry (U.S. Pat. No. 5,012,761; JP 04271754; U.S. Pat. No. 5,133,963; U.S. Pat. No. 5,069,903), and eggs (KR 9311396; U.S. Pat. No. 5,069,903). DHA has also been added as a dietary supplement to infant formula as discussed above, and milk. Sources of DHA for supplementing milk or infant formula include fish products, fatty acid containing microbial oils (U.S. Pat. No. 5,374,657; U.S. Pat. No. 5,397,591; U.S. Pat. No. 5,407,957), or fatty acids extracted from a mixture of egg yolk and coconut oil (U.S. Pat. No. 4,670,285).
Researchers have been able to increase DHA content in the expressed milk of humans (Harris, W. S. et al., Am. J. Clin. Nuir. 40(4): 780-785, 1984; Henderson, R. A., Lipids, 27(11): 863-869, 1992; U.S. Pat. No. 5,069,903), sows (Taugbol, ) O. et al., Zentralbi, Veterinarmed. A. 40(6): 437-443, 1993), and rats (Yonekubo, A., et al. J. Nutr. 123(10): 1703-1708, 1993). However, researchers have had difficulty obtaining significant levels of DHA in cow's milk. (Hebeisen, D. F., et al. Int. J. Vitam. Nutr. Res., 63(3): 229-233, 1993).